The present invention relates to an axially compact axial-flow fan, especially a small-scale fan with a hub diameter of the fan wheel which is at least half as large as the inner diameter of the casing surrounding the fan wheel, which has a cylindrical configuration in its flow passage in the axial central portion and is broadened at least toward the exhaust side by way of corner pockets into a regular multi-corner, for example a square profile, circumscribing the diameter of the fan wheel.
Axial-flow fans of this type are conventional and known through U.S. Pat. No. 3,362,627. In the conventional types of construction, the hub of the fan wheel is fashioned as the motor casing to be essentially cylindrical, wherein the rotor of an external-rotor motor is disposed within this casing, the stator of this motor being mounted to a flange fixedly joined to the casing by way of spokes. Axial-flow fans of this type are primarily installed in electronic appliances to serve as ventilators, where the external dimensions of the casing jacket are predetermined such as by industry standards. On account of the small dimensions--the casing has predominantly square outside faces with a length of about 80 mm.--it is either impossible or possible only at very high expense, for reasons of electric motor technology and manufacturing techniques, to make the diameter of the motor housing smaller than corresponds to about two-thirds and, at the minimum, about one-half the internal diameter of the casing enclosing the fan wheel. This results in relatively small radial lengths for the blades, so that axial-flow fans of this type can hardly be improved upon any more in their efficiency with economical expenditure. Increasing the casing diameter is not possible because of the fixed dimensions. A speed increase leads to a rise in the noise level which in such axial-flow fans must likewise be kept at a minimum, and is furthermore possible only to a minor extent due to the given frequency of the power source.
Additionally, in the electronic appliances which are becoming increasingly more compact, the intake and exhaust chambers upstream and downstream of the axial-flow fans are exceedingly small so that such axial-flow fans frequently operate at very high counterpressures, resulting in reduced air flow rate.
An object of the present invention is to provide an axial-flow fan of the aforementioned type which avoids these disadvantages associated with the known fans and which, specifically, makes it possible to provide an increase in efficiency at least in a partial area of the characteristic output curve, without having to abandon the total concept or having to take complicated measures.
The invention resides in providing the hub of the fan wheel on the inlet side with an annular surface means extending at least over one-third of the entire axial length of the motor and being, for instance, of a conical configuration toward the hub's end face. This feature, which is known per se in axial-flow fans of a different type of construction and is utilized particularly in relatively large axial-flow fans, surprisingly produces the advantage, in conjunction with the special features of the casing design, that the novel axial-flow fan of the present invention provides a markedly larger pressure increase in the region of relatively low air flow rates, so that the fan in this operating range can convey, at higher counterpressures, a larger amount of air. Therefore, especially when installed in appliances of the kind mentioned in the foregoing and when operating, due to the compact structure thereof, against higher counterpressures, the novel axial-flow fan of the present invention is superior to the units known heretofore. In the remaining pressure and air flow range, the novel fan is at least equivalent, and has the further advantage that it produces the increased amount of air in the region of higher static pressures at a markedly lower noise level, which has been proven unequivocally by measuring the noise level with axial-flow fans of a conventional type of structure and with the novel arrangement when installed in a test chamber and operating against increased counterpressure.
It is advantageous to make the average or representative angle of the annular surface means with respect to the axial direction to be 10.degree.-30.degree., preferrably about 20.degree., because in this case an inwardly oriented flaring portion of the inlet region is obtained on an order of magnitude maximal for the fan efficiency. It is also particularly advantageous to arrange only five or seven fan blades uniformly over the periphery of the hub fashioned as the motor housing, which blades may be suitably made of thin steel sheet and are conventionally welded onto the motor housing. If these fan blades are provided over the entire axial length of the motor housing in such a way that they also contact with their inner edges the zone of the inlet region, for instance the conical annular surface at the motor casing, then especially favorable efficiency values are obtained, which may be due to the fact that the free inlet cross section can be optimally enlarged because of the conical configuration of the motor casing, which is all the more true in relation with the very thin end faces of sheet steel fan blades.